1. Field of the Invention
The present invention relates to an optical deflection/scanning apparatus used in an image forming apparatus such as a laser beam printer or a laser facsimile device.
2. Related Background Art
An optical deflection/scanning apparatus used in an image forming apparatus such as a laser beam printer or a laser facsimile apparatus irradiates a light beam such as a laser beam on a rotary polygon mirror, and deflects and scans the beam by high-speed rotation of the rotary polygon mirror. The scanning beam obtained in this manner is formed into an electrostatic latent image on a photosensitive body serving as a recording medium. on a rotary drum. The electrostatic latent image on the photosensitive body is visualized into a toner image by a developing unit. The toner image is transferred to a recording medium such as a recording paper sheet and sent to a fixing unit. The toner on the recording medium is heated and fixed to print the image.
In recent years, as the speed of the optical deflection/scanning apparatus increases, one with a rotary polygon mirror whose rotational speed exceeds 5,000 rpm is developed.
As shown in FIG. 1, a motor for rotating the rotary polygon mirror comprises a rotating shaft 103 supported by an optical box of the optical deflection/scanning apparatus via a ball bearing 102, a rotor 105 made up of a rotor yoke 105a integrally coupled to a flange member 104 integral with the rotating shaft 103 and a rotor magnet 105b, and a stator 107 fixed to a motor board 106. A rotary polygon mirror 101 is pressed against the flange member 104 by an elastic press mechanism 108 made up of a leaf spring 108a, a washer 108b, and a G-ring 108c, and is integrated with the rotating shaft 103 and the rotor 105.
When the stator 107 is excited by a driving current supplied from a driving circuit on the motor board 106, the rotor 105 rotates at a high speed together with the rotary polygon mirror 101 to deflect and scan a light beam irradiated on the rotary polygon mirror 101.
The rotor 105 is constituted by the rotor yoke 105a for ensuring a necessary structure strength (to be described later), and the rotor magnet 105b with magnetic properties. A material for the rotor yoke 105a is generally a metal or a reinforced plastic. As the rotor magnet 105b, a ferrite magnet, or a plastic magnet or so-called rubbernet prepared by kneading a ferrite in a resin and integrally molding them is used. The assembly of the rotor magnet 105b in the rotor yoke 105a is performed by fixing the rotor magnet 105b to the inner circumferential surface of the rotor yoke 105a using an adhesive, or press-fitting the rotor magnet 105b inside the rotor yoke 105a. 
Upon rotating the motor at a high speed, a large centrifugal force is generated in the rotor. If the rotor does not have a strength large enough to stand this force, the rotor may fracture, be divided, and scatter during rotation. Since the rotor magnet made of a rubbernet, a plastic magnet, or the like cannot be expected to have such a large strength, the rotor magnet is attached to the inside of the rotor yoke having a sufficient strength to form a strong structure which can stand the centrifugal force.
According to the prior art, however, even if the rotor magnet is attached to the inside of the rotor yoke having a sufficient strength, the lower end of the rotor magnetic easily cracks. This may lead division and fracture of the rotor magnet.
More specifically, as shown in FIG. 2A, the rotor yoke 105a is generally made of a sheet metal. A curved portion R0 is formed at the lower end of the rotor yoke 105a as a press sag in processing the sheet metal. The outer circumferential surface of the lower end of the rotor magnet 105b cannot tightly contact the rotor yoke 105a, and a gap is made between the rotor yoke 105a and the rotor magnet 105b. When the motor rotates in this state, a centrifugal force A directly acts on the lower end of the rotor magnet 105b. As shown in FIG. 2B, a tensile stress is generated on the inner circumferential surface of the rotor yoke 105a near the start of the curved portion R0 of the rotor yoke 105a, i.e., at one end at a center angle xcex80. Such portions where the tensile stress is generated successively exist in the circumferential direction at the lower end of the rotor magnet 105b. If the tensile stress exceeds the allowable stress of a material for the rotor magnet 105b, a crack S0 is formed. If the crack S0 grows and reaches the outer circumferential surface of the rotor magnet 105b, the lower end of the rotor magnet 105b is divided and fractures. As a result, the lower end of the rotor magnet 105b is lost, the rotation unbalance of the rotor 105 becomes very large, and large vibrations and noise are generated upon high-speed rotation.
FIG. 3 shows the example wherein a flange portion F0 is formed at the lower end of a rotor yoke 205a, and a balance weight W0 is attached to the flange portion F0 in order to correct the rotation unbalance of a rotor 205 (balance correction). In this case, since the lower end of the rotor yoke 205a is bent, a curved portion R0 having a center angle xcex80 larger than that in the example of FIGS. 2A and 2B is easily formed. Therefore, the lower end of a rotor magnet 205b cracks and fractures more easily.
FIGS. 4A and 4B show the example using a rubbernet as a rotor magnet 305b. The rubbernet is rolled by a roller during the manufacturing process. Small antislip projections are formed on the surface of the roller, and dimples D0 are formed on the surface of the rotor magnet 305b by rolling.
Since the dimple D0 is a recess having a quadrangular pyramid shape. Upon generation of the above tensile stress, the stress concentrates at the dimple D0, and the crack S0 is easily formed. Since the dimples D0 are aligned near each other in the circumferential direction at an equal interval, a crack so growing from the edge of a dimple D0 in the circumferential direction is connected to an adjacent crack. In this manner, the cracking and fracture of the lower end of the rotor magnet 305b progress at a breath.
The present invention has been made in consideration of the above problems of the prior art, and has as its object to provide an optical deflection/scanning apparatus in which the speed of a rotary polygon mirror can be greatly increased while preventing troubles such as separation and fracture of the lower end of the rotor of a motor by a centrifugal force during the rotation of the rotary polygon mirror.
To achieve the above object, according to the present invention, there is provided an optical deflection/scanning apparatus comprising a rotary polygon mirror for reflecting a light beam, and a motor for rotating the rotary polygon mirror, the motor having a rotor yoke with a curved portion at one end, and a rotor magnet attached to an inside of the rotor yoke, wherein the curved portion of the rotor yoke projects from one end of the rotor magnet in a direction of height.
The optical deflection/scanning apparatus may comprise a rotary polygon mirror for reflecting a light beam, and a motor for rotating the rotary polygon mirror, the motor having a rotor yoke with a curved portion at one end, and a rotor magnet attached to an inside of the rotor yoke, wherein one end of the rotor magnet is made locally thin at the same level as the curved portion of the rotor yoke.
The optical deflection/scanning apparatus may comprise a rotary polygon mirror for reflecting a light beam, and a motor for rotating the rotary polygon mirror, the motor having a rotor yoke with a curved portion at one end, and a rotor magnet attached to an inside of the rotor yoke, wherein a gap between one end of the rotor magnet and the curved portion of the rotor yoke is filled with a filler.
The optical deflection/scanning apparatus may comprise a rotary polygon mirror for reflecting a light beam, and a motor for rotating the rotary polygon mirror, the motor having a rotor yoke with a curved portion at one end, and a rotor magnet attached to an inside of the rotor yoke, wherein an array of dimples of the rotor magnet is arranged at a remaining portion except for one end at the same level as the curved portion of the rotor yoke.
The optical deflection/scanning apparatus may comprise a rotary polygon mirror for reflecting a light beam, and a motor for rotating the rotary polygon mirror, the motor having a rotor yoke with a curved portion at one end, and a rotor magnet attached to an inside of the rotor yoke, wherein an alignment direction of an array of dimples of the rotor magnet is inclined at a predetermined angle with respect to a circumferential direction of the rotor magnet.
The optical deflection/scanning apparatus may comprise a rotary polygon mirror for reflecting a light beam, and a motor for rotating the rotary polygon mirror, the motor having a rotor yoke with a curved portion at one end, and a rotor magnet attached to an inside of the rotor yoke, wherein an array of dimples of the rotor magnet is perpendicular to a circumferential direction of the rotor magnet.
If the lower end of the rotor magnet overlaps the curved portion of the lower end of the rotor yoke, a gap is formed between them. The lower end of the weak rotor magnet cannot be supported, is cracked by the centrifugal force during the rotation of the motor, and divided and fractured. For this reason, by setting the attaching position of the rotor magnet high so as to make the curved portion of the rotor yoke project from the lower end of the rotor magnet, the entire outer surface of the rotor magnet tightly contacts the rotor yoke. With this structure, the gap between the rotor magnet and the rotor yoke is eliminated, and, for example, the rotor magnet will not fracture by the centrifugal force. By preventing troubles caused by the centrifugal force, the speed of the optical deflection/scanning apparatus can greatly increase.
If the optical deflection/scanning apparatus comprises a rotary polygon mirror for reflecting a light beam, and a motor for rotating the rotary polygon mirror, the motor has a rotor yoke with a curved portion at one end, and a rotor magnet attached to the inside of the rotor yoke, and one end of the rotor magnet is made locally thin at the same level as the curved portion of the rotor yoke, the mass of the rotor magnet can be locally reduced at a portion where the rotor magnet overlaps the curved portion of the rotor yoke, thereby preventing a large centrifugal force from acting on this portion. Accordingly, for example, the rotor magnet will not fracture by the centrifugal force.
If the optical deflection/scanning apparatus comprises a rotary polygon mirror for reflecting a light beam, and a motor for rotating the rotary polygon mirror, the motor has a rotor yoke with a curved portion at one end, and a rotor magnet attached to the inside of the rotor yoke, and the gap between one end of the rotor magnet and the curved portion of the rotor yoke is filled with a filler, the rotor magnet can be satisfactorily supported via the filler between the curved portion of the rotor yoke and the rotor magnet. The cracks and the like will not be formed by the centrifugal force.
If the optical deflection/scanning apparatus comprises a rotary polygon mirror for reflecting a light beam, and a motor for rotating the rotary polygon mirror, the motor has a rotor yoke with a curved portion at one end, and a rotor magnet attached to the inside of the rotor yoke, and an array of dimples of the rotor magnet is arranged at a remaining portion except for one end at the same level as the curved portion of the rotor yoke, for example, the rotor magnet will not fracture by the centrifugal force because no dimple at which the tensile stress concentrates exists at the same level as the curved portion of the rotor yoke.
If the optical deflection/scanning apparatus comprises a rotary polygon mirror for reflecting a light beam, and a motor for rotating the rotary polygon mirror, the motor has a rotor yoke with a curved portion at one end, and a rotor magnet attached to the inside of the rotor yoke, and the alignment direction of an array of dimples of the rotor magnet is inclined at a predetermined angle with respect to the circumferential direction of the rotor magnet, the pitch of the dimple at which the tensile stress concentrates is increased in the circumferential direction of the rotor magnet. As a result, the cracks formed at the dimples will not be connected to prevent, e.g., the fracture of the rotor magnet.
If the optical deflection/scanning apparatus comprises a rotary polygon mirror for reflecting a light beam, and a motor for rotating the rotary polygon mirror, the motor has a rotor yoke with a curved portion at one end, and a rotor magnet attached to the inside of the rotor yoke, and the alignment direction of an array of dimples of the rotor magnet is perpendicular to the circumferential direction of the rotor magnet, the longitudinal direction of the dimple at which the tensile stress concentrates is made incoincident with the circumferential direction of the rotor magnet. With this arrangement, a large crack at the dimple will not be formed to prevent, e.g., the fracture of the rotor magnet.